The present invention relates to electrochemical devices and, more particularly, to a method of fabricating solid, non-aqueous electrochemical cells demonstrating improved performance.
Non-aqueous lithium electrochemical cells typically include an anode, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the lithium anode.
Recently, the lithium metal anode has been replaced with a carbon anode such as coke or graphite intercalated with lithium ions to form LixC. In operation of the cell, lithium passes from the carbon through the electrolyte to the cathode where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode where it reintercalates into the carbon. Because no metallic lithium is present in the cell, melting of the anode does not occur even under abuse conditions. Also, because lithium is reincorporated into the anode by intercalation rather than by plating, dendritic and spongy lithium growth does not occur. Non-aqueous lithium electrochemical cells are discussed in U.S. Pat. Nos. 4,472,487, 4,668,595 and 5,028,500.
Despite these advantages, present methods of fabricating electrochemical cells produce cells and batteries that are prone to short circuits. This occurs, for instance, when a current collector develops burrs or rough edges during fabrication. These burrs can come into contact with another current collector. Simply reducing the size of the current collectors is not a desirable option of avoiding short circuits as this also reduces the electrical contact area between the current collector and electrode material. In addition, this technique may be difficult to implement with present methods of fabricating electrochemical cells. For instance, where electrochemical cells (or precursors thereof) are mass produced (e.g., cut to size) from a web comprising a tri-layer laminate having (1) an anode layer, (2) an electrolyte (or separator) layer, and (3) a cathode layer, it would be difficult to reduce the size of the current collectors within the anode and cathode in this process.
Short circuits are also caused by misplacement or misalignment during assembly. For example, in the placement of each component piece upon the underlying pieces, prior to lamination or welding, any misplacement, or skewing of the topmost piece can, later, result in shorting if a small portion of the edge of one electrode becomes positioned such that it can touch a portion of the opposite electrode, especially due to pressure, flexure or movement caused by thermal expansion or contraction.
The present invention is based in part on a process of fabricating electrochemical cells and batteries wherein the successive anode and cathode layers are separated by a polymeric electrolyte layer having a protruding polymer edge or strip around its perimeter which reduces the likelihood of inadvertent contact between adjacent anode and cathode current collectors. The polymer edge functions as a non-conducting physical barrier.
In one aspect, the invention is directed to a method of preparing an electrochemical cell comprising the steps of:
(a) forming an anode film onto at least one surface of an anode current collector to form an anode;
(b) forming a cathode film onto at least one surface of a cathode current collector to form a cathode; and
(c) interposing a polymeric layer containing an electrolyte solution between the anode and cathode wherein the polymeric layer has a surface area that is larger than that of the anode film and cathode film, wherein the perimeter of the polymeric electrolyte layer forms an exposed strip that serves as a barrier to direct contact between the anode current collector and cathode current collector.
In another aspect, the invention is directed to a method of preparing an electrochemical cell comprising the steps of:
(a) preparing a polymeric layer comprising a first plasticizer;
(b) covering at least one surface of an anode current collector with a layer of anode material comprising a first polymer, an intercalation carbon material, and a second plasticizer to form an anode precursor wherein each layer of said anode material has a surface area that is smaller than that of the polymeric layer;
(c) covering at least one surface of a cathode current collector with a layer of cathode material comprising a second polymer, a cathode active material, and a third plasticizer to form a cathode precursor wherein each layer of said cathode material has a surface area that is smaller than that of the polymeric layer;
(d) interposing the polymeric layer between the anode precursor and the cathode precursor so that the perimeter of the polymeric layer forms an exposed strip that serves as a barrier to direct contact between the anode current collector and cathode current collector;
(e) removing said first, second, and third plasticizer; and
(f) placing an electrolyte solution comprising an electrolyte solvent and an inorganic salt into said anode precursor, cathode precursor, and polymeric layer.
In a further aspect, the invention is directed to an electrochemical cell comprising:
an anode having an anode current collector with an anode film on at least one side of the anode current collector;
a cathode having a cathode current collector with a cathode film on at least one side of the cathode current collector; and
a polymeric layer containing an electrolyte solution comprising an electrolyte solvent and a salt that is interposed between the anode and cathode, wherein the polymeric layer has a surface area that is larger than that of the anode film and cathode film, wherein the perimeter of the polymeric layer forms an exposed strip that serves as a barrier to direct contact between the anode current collector and cathode current collector.
In accordance with still another aspect of the present invention, an apparatus for preparing an electrochemical cell is provided. The apparatus includes a first laminating station, an anode current collector being laminated to at least one anode material film on at least one side of the anode current collector to form an anode precursor at the first laminating station, the anode current collector including an extending anode tab extending past an edge of the anode material film, and the anode material film including a first polymer, an intercalation carbon material, and a first plasticizer. The apparatus further includes a second laminating station, a cathode current collector being laminated to at least one cathode material film on at least one side of the cathode current collector to form a cathode precursor at the second laminating station, the cathode current collector including an extending cathode tab extending past an edge of the cathode material film, and the cathode material film including a second polymer, a cathode active material, and a second plasticizer. The apparatus further includes an assembling station, a polymeric layer including a third plasticizer being interposed between the anode precursor and the cathode precursor at the assembling station such that the polymeric layer prevents direct contact between the anode current collector and the cathode current collector. The apparatus further includes a fusion station, the polymeric layer being fused to the anode precursor and the cathode precursor at the fusion station to form a bicell battery. The apparatus further includes a stacking station, a plurality of bicell batteries being stacked on top of one another at the stacking station to form a stack. The apparatus further includes a welding station, a plurality of anode tabs of the stack being welded to one another and to a conductive anode lead, and a plurality of cathode tabs of the stack being welded to one another and to a conductive cathode lead at the welding station. The apparatus further includes an extraction station, downstream from the welding station, the first, second, and third plasticizers being extracted from the stack at the extraction station. The apparatus further includes an installation station, downstream from the extraction station, the stack being installed in a recess in a stack receptacle such that at least portions of the anode lead and the cathode lead extend beyond an outer periphery of the stack receptacle. The apparatus further includes a filling station, an electrolyte solution including an electrolyte solvent and an inorganic salt being filled into the receptacle and into the anode precursor, the cathode precursor, and the polymeric layer at the filling station. The apparatus further includes a closing station, a top being closed over the recess in the stack receptacle such that the portions of the anode lead and the cathode lead are disposed outside of an outer periphery of the closed stack receptacle.
In accordance with still another aspect of the present invention, an apparatus for preparing an electrochemical cell is provided. The apparatus includes an assembling station, a polymeric layer including a first plasticizer being interposed between (a) an anode precursor, the anode precursor including an anode current collector laminated to at least one anode material film on at least one side of the anode current collector, the anode material film including a first polymer, an intercalation carbon material, and a second plasticizer, and (b) a cathode precursor, the cathode precursor including a cathode current collector laminated to at least one cathode material film on at least one side of the cathode current collector, the cathode material film including a second polymer, a cathode active material, and a third plasticizer, at the assembling station such that the polymeric layer prevents direct contact between the anode current collector and the cathode current collector. The apparatus further includes an extraction station, downstream from the assembling station, the first, second, and third plasticizers being extracted from the anode precursor, the cathode precursor, and the polymeric layer, respectively, at the extraction station. The apparatus further includes a filling station, an electrolyte solution including an electrolyte solvent and an inorganic salt being filled into the anode precursor, the cathode precursor, and the polymeric layer at the filling station.
In accordance with another aspect of the invention, an apparatus for activating an electrochemical cell is described. The apparatus includes a first filling station, a first amount of an electrolyte solution including an electrolyte solvent and an inorganic salt being filled into a receptacle for one or more bicell batteries such that the electrolyte solution is absorbed into pores in the anode precursor, the cathode precursor, and the polymeric layer of the one or more bicell batteries, from which first, second, and third plasticizers, respectively, have been extracted, at the first filling station. The apparatus further includes one or more subsequent filling stations disposed downstream from the first filling station. Subsequent amounts of the electrolyte solution are filled into the receptacle at the subsequent filling stations such that the electrolyte solution is absorbed into pores in the anode precursor, the cathode precursor, and the polymeric layer at the subsequent filling stations. The subsequent amounts of the electrolyte solution added at each subsequent filling station is no more than equal to the amount added at a preceding one of the first filling station and the subsequent filling stations. The apparatus includes means for transporting the one or more bicell batteries disposed in the recess in the receptacle to and from the first filling station and to and from the one or more subsequent filling stations.